1. Field of the Invention
This invention relates to a miniaturized spark plug having improved fouling resistance.
2. Description of the Related Art
In recent years, with the improvement of the performance of engines to higher levels, the construction of an engine head has become complicated. Also, the available space for fixing a spark plug used to ignite an internal combustion engine, such as an automobile gasoline engine and the like, has decreased. Therefore, the development of a miniaturized spark plug has been in great demand. The miniaturization of a spark plug involves a reduction in the diameter of a main metal member (metallic shell) on which a mounting portion with respect to an engine head is formed. However, a diameter of an insulator inserted through an inner side of the main metal member cannot carelessly be reduced in view of the necessity of maintaining the voltage resistance of the spark plug.
The diameter of a front end portion of an insulator of a related art spark plug is reduced due to the provision of a stepped portion formed thereon, and the insulator is combined with a main metal member with the stepped portion engaged with a projection formed on an inner circumferential surface of the main metal member. Therefore, in order to reduce the diameter of the main metal member in such a structure, a method of reducing the clearance width between the inner circumferential surface of the projection of the main metal member and the outer circumferential surface of the insulator opposed thereto is employed. This is because there is a limit to the reduction of the outer diameter of the insulator.
3. Problems to Be Solved by the Invention
However, when the width of the clearance is reduced, the fouling resistance of the spark plug is deteriorated. Namely, when the spark plug is used in a low-temperature environment of an electrode temperature of not higher than 450xc2x0 C., it generates a large amount of unburnt gas. When such an unburnt gas generating condition continues for a long period of time during, for example, predelivery of a gaseous mixture, the insulator is placed in a so-called xe2x80x9csmokingxe2x80x9d or xe2x80x9cfoggingxe2x80x9d condition. As a result, the surface of the insulator inside the metal member is contaminated with a conductive substance, such as carbon, etc., and imperfect operation of the insulator is liable to occur. Especially, when the surface of the insulator is contaminated in the above-mentioned clearance due to entry of unburnt gas thereinto, spark discharge occurs in the clearance, and normal ignition cannot be sustained.
The present invention has been made in view of the above problems of the prior art, and an object of the present invention is to provide a spark plug having a structure that is suitably miniaturized without impairing the fouling resistance thereof.
The above object of the present invention has been achieved by providing a spark plug having a center electrode 3, an insulator 2 provided on the outer side of the center electrode 3, a cylindrical main metal member 1 provided on the outer side of the insulator 2, and an earth electrode 4 which is provided so that the earth electrode is combined at one end portion thereof with the main metal member 1 and opposed at the other end portion thereof to a free end of the center electrode 3, and which forms a spark discharge gap g between the earth electrode and center electrode. The spark plug has a front side at which the spark discharge gap g is positioned with respect to an axial direction O of the insulator 2 with the other side being a rear side, characterized in that the insulator 2, a diameter of a front end portion 2i of which is reduced by a circumferentially extending stepped portion thereof provided as an insulator-side locking portion 2h, is inserted into the main metal member from a rear opening thereof. The insulator-side locking portion 2h is engaged with a metal member-side locking portion 1c projecting from an inner circumferential surface of the main metal member with an outer circumferential surface (clearance-forming outer circumferential surface) 2k of the portion 2i positioned ahead of the locking portion 2h of the insulator 2 opposed to an inner circumferential surface (clearance-forming inner circumferential surface) 52 so as to form in a locking position a clearance Q of a predetermined amount therebetween. Furthermore, an amount xcex2 of the clearance in the locking position is expressed by the equation:
xcex2=(D1xe2x88x92d1)/2xe2x80x83xe2x80x83(1) 
wherein d1 represents an outer diameter of the clearance-forming outer circumferential surface 2k; and D1 represents an inner diameter of the clearance-forming inner circumferential surface 52, where xcex2 is not greater than 0.4 mm but not smaller than 0.05 mm.
When the difference D1xe2x88x92d1 between the outer diameter d1 of the clearance-forming outer circumferential surface and the inner diameter D1 of the clearance-forming inner circumferential surface differs depending upon the axial position, the amount xcex2 of a clearance in the locking position is represented by a value obtained at a position in which the diameter difference becomes minimal. Although the metal member-side locking portion can be formed of, for example, an annular projection, it is not limited to this mode as long as it can function as a locking portion.
In order to reduce the outer diameter of the main metal member without impairing the voltage resisting characteristics of the spark plug as described above, the wall thickness of the insulator cannot be greatly reduced. Thus, the amount xcex2 of clearance in the locking position is necessarily reduced. However, setting a value of P to the highest possible level so as to prevent the generation of jumping sparks in this clearance when the spark plug is fouled has heretofore been the conventional approach. Therefore, reducing the amount xcex2 of the clearance in the locking position to meet a demand for miniaturizing a spark plug has heretofore been considered to be problematic in view of the necessity of preventing the occurrence of jumping sparks when the spark plug is fouled.
The present inventors have carefully studied the amount xcex2 of the clearance in the locking position to discover that, when this amount is positively reduced to less than a certain limit (where conventionally at least 0.5 mm was thought to be necessary), the fouling resistance of the spark plug is unexpectedly improved to a remarkable extent, and jumping sparks occurring in the clearance in the locking position when the spark plug is fouled can be prevented. The present invention was thus completed based on these findings. More concretely, when the amount xcex2 of the clearance in the locking position is set to not higher than 0.4 mm, entry of unburnt gas into the clearance in the locking position can be reliably blocked, and contamination of the insulator surface in the clearance in the locking position can be prevented. As a result, spark plug miniaturization can be effectively attained without impairing the fouling resistance thereof.
When the amount xcex2 of the clearance in the locking position exceeds 0.4 mm, it becomes difficult to prevent entry of an unburnt gas into the clearance. Thus, it becomes impossible to prevent contamination of the insulator surface in the clearance in the locking position. When the amount xcex2 of the clearance in the locking position becomes extremely small, contaminants do not enter into the clearance in the locking position. However, when contaminants are deposited on the portion of the insulator surface which extends forward of the clearance in the locking position, a layer of accumulated contaminant contacts the locking portion of the main metal member positioned on the opposite side thereof via the clearance in the locking position, and is liable to cause a short-circuit to occur. Consequently, ignitability of the spark plug may be impaired in some cases. Giving consideration to this point, it is preferable to set the amount xcex2 of the clearance in the locking position to not smaller than 0.05 mm, and more preferably not smaller than 0.2 mm.
In another aspect of the invention, this clearance Q needs a clearance distance (xcex2L) extending in the locking position, which means that an annular space defined by the clearance amount (xcex2) measured in a radial direction of the spark plug and the clearance distance (xcex2L) measured in an axial direction of the spark plug is incorporated between an inner circumferential surface 52 of the main metal member 1 and an outer circumferential surface 2k of the insulator 2 (in reference to the encircled drawing in FIG. 1). In other words, the clearance amount (xcex2) of 0.05-0.4 mm should continue or be maintained for a distance or length of at least 0.5 mm in the axial direction so as to attain effective protection of the clearance interior from fouling. However, if the clearance distance (QL) exceeds 2.5 mm, deposits such as carbon are liable to accumulate on the insulator around the clearance Q, causing jumping-sparks there. Therefore, the clearance distance should be 0.5-2.5 mm so long as the clearance amount (xcex2) (or rather width) of 0.05-0.4 mm is maintained over that distance. The best fouling resistance for the spark plugs is attained when the above mentioned circumferential surfaces forming the annular space run in parallel in a distance of 1-2.5 mm by maintaining a clearance amount of 0.2-0.4 mm. As a result, a miniaturized spark plug can spark without impairing the fouling resistance thereof.